Known nuclear reactors, such as pressurized water reactors, include control rods that contain neutron absorbers with varying absorption capability for shutdown or power level control and sometimes also non-neutron absorbing materials to first breed plutonium and to later burn it as a fuel component.
Generally, these rods are mounted, via their upper end, to an assembly, or spider, which supports a plurality of such rods from simultaneous movement into and out of the reactor active core region. While in the core region, the control rods enter fuel assembly thimbles.
The assembly of a spider and a plurality of control rods may be designated a rod cluster control or a water displacer rod assembly, depending on the function to be performed by the control rods.
During reactor operation, the control rods are withdrawn from the active core region by lifting the assembly into the upper internals of the reactor pressure vessel. Movements of the assembly are guided by a guide tube presenting a plurality of guide sections, or cards, which contact the rods in respective regions. The number of guide sections provided for each control rod is selected as a compromise between the desire to reduce drag forces, which entails a minimum number guide sections, and the need to reduce the distance between guide sections in order to limit the amplitude of flow-induced vibration forces.
Drag forces can be controlled by design approaches which limit the drag forces, and thus reduce drag friction, by a hydraulic pressure balance.
The drag which does exist is a source of wear along each rod surface which slides along a guide section. In addition, when the rods are being maintained in their raised position in the guide tube, they experience movement relative to the guide sections due to flow induced vibration, which movement is a further cause for wear.
Such wear can influence the useful life of the control rods.
The rate of such wear is partly determined by the materials of the control rods and guide sections. The latter are usually made of stainless steel, while neutron absorber control rods are generally sheathed in stainless steel and water displacer rods are sheathed in a non-neutron absorbing material such as zircalloy.
Thus, wear improsed on neutron absorbing rods occurs primarily during insertion and withdrawal movements. In the case of water displacer rods, movements are limited, but since the material is softer, these rods are more susceptible to wear due to flow induced vibration while the rods are in their raised, or parked, position.
Once the outer wall of a control rod has been worn to a certain depth at one location, the rod must be replaced. Therefore, if such wear can be retarded, the useful life of the spider-rod assembly can be increased.